Diamondus Beta is one of the few available tilesets that was actually rejected and destroyed (other nonavailable tilesets in this field include Carrotsville, and most stuff from Cyberspaz), so it's hard to imagine that it would be the best.
I'm not sure if I have a favorite. I like Heaven (best Agama), Jungle (just needs to have some fixed masking), Diamondus (see other topic asking same question), Carrotus (general requirement for plot-based packs), and beyond that I don't tend to reuse tilesets all that much. Mez01 is always good and through a strange series of events I tend to use HocusEP1 over and over.